10,000 feet msl or above — Unrestricted (but not more than Mach 1) Below 10,000 feet msl — 250 kias. Class B airspace — Unrestricted at or above 10,000 feet msl, 250 kias below 10,000 feet msl. Beneath Class B or in VFR corridor through Class B — 200 kias.

In the United States and many other jurisdictions, there is no specific numerical speed limit once an aircraft climbs above 10,000 feet Mean Sea Level (MSL), other than the physical limitations of the aircraft itself (Vmo/Mmo). The well-known "250-knot rule" only applies to aircraft operating below 10,000 feet. Once a pilot crosses that "10,000-foot line," they typically accelerate to their "clean" climb speed, which for most commercial jets is between 280 and 320 knots (or a specific Mach number, like Mach 0.78 to 0.85). However, there is a secondary restriction: all aircraft must remain subsonic (below Mach 1.0) over land in the U.S. unless they have a specific supersonic flight authorization. Additionally, Air Traffic Control (ATC) may impose "flow control" speed restrictions at any altitude to maintain spacing between aircraft. So, while you can technically go as fast as your engines allow above 10,000 feet, the "limit" is usually a combination of fuel efficiency, the speed of sound, and the instructions of the controller managing the busy high-altitude "highways" of the sky.